Flocculation


Flocculation, in the field of chemistry, is a process in which colloids come out of suspension in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution. In the flocculated system, there is no formation of a cake, since all the flocs are in the suspension.
Coagulation and flocculation are important processes in water treatment with coagulation to destabilize particles through chemical reaction between coagulant and colloids, and flocculation to transport the destabilized particles that will cause collisions with floc.

Term definition

According to the IUPAC definition, flocculation is "a process of contact and adhesion whereby the particles of a dispersion form larger-size clusters". Flocculation
is synonymous with agglomeration and coagulation / coalescence.
Basically, coagulation is a process of addition of coagulant to destabilize a stabilized charged particle. Meanwhile, flocculation is a mixing technique that promotes agglomeration and assists in the settling of particles. The most common used coagulant is alum, Al23 • 14 H2O.
The chemical reaction involved:
Al23 • 14 H2O → 2 Al3 + 6 H+ + 3 SO42- + 8 H2O
During flocculation, gentle mixing accelerates the rate of particle collision, and the destabilized particles are further aggregated and enmeshed into larger precipitates. Flocculation is affected by several parameters, including mixing speeds, mixing intensity, and mixing time. The product of the mixing intensity and mixing time is used to describe flocculation processes.

Applications

Surface chemistry

In colloid chemistry, flocculation refers to the process by which fine particulates are caused to clump together into a floc. The floc may then float to the top of the liquid, settle to the bottom of the liquid, or be readily filtered from the liquid. Flocculation behavior of soil colloids is closely
related to freshwater quality. High dispersibility of soil colloids not only directly causes turbidity of the surrounding water but it also induces eutrophication due to the adsorption of nutritional substances in rivers
and lakes and even boats under the sea.

Physical chemistry

For emulsions, flocculation describes clustering of individual dispersed droplets together, whereby the individual droplets do not lose their identity. Flocculation is thus the initial step leading to further ageing of the emulsion. Flocculation is used in mineral dressing, but can be also used in the design of physical properties of food and pharmaceutical products.

Civil engineering/earth sciences

In civil engineering, and in the earth sciences, flocculation is a condition in which clays, polymers or other small charged particles become attached and form a fragile structure, a floc. In dispersed clay slurries,
flocculation occurs after mechanical agitation ceases and the dispersed clay platelets spontaneously form flocs because of attractions between negative face charges and positive edge charges.

Biology

Flocculation is used in biotechnology applications in conjunction with microfiltration to improve the efficiency of biological feeds. The addition of synthetic flocculants to the bioreactor can increase the average particle size making microfiltration more efficient. When flocculants are not added, cakes form and accumulate causing low cell viability. Positively charged flocculants work better than negatively charged ones since the cells are generally negatively charged.

Cheese industry

Flocculation is widely employed to measure the progress of curd formation in the initial stages of cheese making to determine how long the curds must set. The reaction involving the rennet micelles are modeled by Smoluchowski kinetics. During the renneting of milk the micelles can approach one another and flocculate, a process that involves hydrolysis of molecules and macropeptides.
Flocculation is also used during cheese wastewater treatment. Three different coagulants are mainly used:
In the brewing industry flocculation has a different meaning. It is a very important process in fermentation during the production of beer where cells form macroscopic flocs. These flocs cause the yeast to sediment or rise to the top of a fermentation at the end of the fermentation. Subsequently, the yeast can be collected from the top or the bottom of the fermenter in order to be reused for the next fermentation.
Yeast flocculation is primarily determined by the calcium concentration, often in the 50-100ppm range. Calcium salts can be added to cause flocculation, or the process can be reversed by removing calcium by adding phosphate to form insolubable calcium phosphate, adding excess sulfate to form insoluble calcium sulfate, or adding EDTA to chelate the calcium ions. While it appears similar to sedimentation in colloidal dispersions, the mechanisms are different.

Water treatment process

Flocculation and sedimentation are widely employed in the purification of drinking water as well as in sewage treatment, storm-water treatment and treatment of industrial wastewater streams. Typical treatment processes consist of grates, coagulation, flocculation, sedimentation, granular filtration and disinfection.

Jar test

The purpose of this test is to select types of coagulant and also to estimate the optimal dose needed in removing the
charged particles that occurred in raw water. Jar test is an experiment to understand the processes of coagulation, flocculation and sedimentation.
Jar test apparatus consists of six batch beakers, and equipped with a paddle mixer for each beaker. In a standard practice, jar test involves rapid mixing, followed by slow mixing and later the sedimentation process.

Deflocculation

Deflocculation is the exact opposite of flocculation, also sometimes known as peptization. Sodium silicate is a typical example. Usually in higher pH ranges in addition to low ionic strength of solutions and domination of monovalent metal cations the colloidal particles can be dispersed.
The additive that prevents the colloids from forming flocs is called a deflocculant. For deflocculation imparted through electrostatic barriers, the efficacy of a deflocculant can be gauged in terms of zeta potential. According to the Encyclopedic Dictionary of Polymers deflocculation is "a state or condition of a dispersion of a solid in a liquid in which each solid particle remains independent and unassociated with adjacent particles. A deflocculated suspension shows zero or very low yield value".
Deflocculation can be a problem in wastewater treatment plants as it commonly causes sludge settling problems and deterioration of the effluent quality.